For obvious reasons, it is essential for an aircraft flight crew to accurately know the quantity of fuel remaining in the fuel tanks of its aircraft at all times, but particularly near the end of a flight when the amount of fuel remaining may effect critical decisions. In addition, the cost of carrying large amounts of unnecessary fuel is high and reduces the amount of transportable cargo. Most large commercial aircraft have a plurality of fuel tanks, with the majority of the fuel being carried in wing tanks, so that its mass is concentrated at the center of lift. Typically, each tank on the aircraft includes a capacitance fuel quantity measurement system that drives a cockpit display indicating the mass of fuel remaining in the tank.
Determination of the quantity of fuel in a tank by a capacitance fuel quantity measurement system is a function of the dielectric constant of the fuel, which in turn depends on the temperature and density of the fuel mixture. A capacitance fuel quantity measurement system includes a plurality of capacitance probes, which are usually mounted in the bottom of each tank, along with a dielectric constant compensator that is used to measure the dielectric constant of the fuel. Since the compensator is also disposed at the bottom of the tank, it measures the dielectric constant of the highest density fuel instead of determining that parameter for the average density of fuel in the tank. In addition, water in the fuel is found in higher concentration in the bottom layer of fuel, where its effect on the compensation of the dielectric constant is greatest. The combined error from these two factors may range from two percent to four percent. Contamination caused by fungus or dirt adhering to the capacitance probes and the effect of contamination on the measurement of the dielectric constant by the dielectric compensator may introduce an additional one to two percent error.
Since the density of the fuel impacts the fuel mass measurement, significant temperature gradients between the top and bottom layers of fuel can produce up to a five percent error in the measurement. No provision is made in existing systems for measuring the temperature at different elevations in each tank to compensate for the varying density distribution caused by temperature stratification.
Capacitance probes and compensators are normally sealed inside each tank, making maintenance more difficult because the electrical wiring and probes are in the wetted fuel tank volume and can only be removed after the tank is drained. Rework and calibration of the capacitive system may require up to three days of airplane out-of-service time.
In attempting to overcome the accuracy and maintenance problems associated with the conventional capacitance fuel measurement system, a method and apparatus for determining the density, volume, and mass of fuel in an aircraft fuel tank as a function of pressure has been developed, as disclosed in U.S. Pat. No. 4,553,216. In this system, four pressure sensors are mounted in an array at unequal depths below the surface of the fuel in a tank. The four pressure signals produced by the transducers are processed by a microcomputer to determine the fuel density and the orientation of the fuel surface with respect to its distance from each sensor. Given the orientation of the fuel surface (and the known dimensional characteristics of the tank), the volume and mass of fuel in the tank may be computed. Alternatively, if the fuel density is either known or otherwise measured, the volume and mass of fuel may be determined using only three pressure sensors.
The approach used in the referenced patent to determine fuel density volume and/or mass as a function of pressure overcomes some of the problems of the more typical capacitance measurement system; however, it does not compensate for varying density of the fuel due to temperature gradients or for variation in the pressure caused by acceleration in a direction normal to the surface of the fuel. Any change in direction or velocity of an aircraft from level flight can produce an acceleration having a component normal to the fuel surface that will affect the measurement of pressure. Failure to compensate for such acceleration can produce significant, instantaneous errors that require substantial time to average out if time filtering is used.
A further problem with the prior art pressure sensor system for measuring fuel quantity results from the disposition of the pressure sensor array in the tank. To ensure that the pressure sensors are submerged as fuel is consumed, they must be mounted at the lowest point in the tank. However, during certain aircraft maneuvers when the fuel level is low, the pressure sensor array may not be submerged. For example, as a plane accelerates or climbs, the resulting acceleration may cause the fuel in a wing tank to flow toward the wing trailing edge and tip, leaving the pressure sensor array exposed. Furthermore, the computation of fuel volume and mass remaining in the tank as a function of pressure is affected by the changes in tank shape that occur due to wing flexure while the aircraft is in flight; yet, there is no provision in the above-noted prior art patent to compensate for this variable.